The formation of intricately or irregularly shaped parts, such as mold cavities for the injection molding of thermoplastics, gears, sprockets, or threaded parts, may involve many processing steps. Often the formation process involves the steps of casting, machining, heat treatment for hardening, tempering, polishing and plating of the parts. Significant volume changes during casting may require expensive and careful processing. Machining and polishing can be particularly difficult for intricate or irregular shapes. The formation of parts from very hard, corrosion resistant alloys greatly increases the difficulty in processing.
Multiple cavities are often formed in a part by hobbing or cold forming of the metal surface of the part. Pressure from a harder metallic replica of a desired part (hob) may be applied to form the mold cavity. However, this process requires the use of a softer metal, such as low carbon steel, and case hardening and polishing of the cast part.
The molding of parts from metal powder may also be accomplished by "press and sinter metallurgy", which comprises the steps of filling a mold with a powder, compacting the powder, and heating the powder to cause the particles to bond together by sintering. This process is impractical for the formation of complex parts because it is difficult to achieve a uniform density of the powder in irregular mold shapes.
In the "powder-injection molding" process, a binder/powder paste mixture is formulated in which the binder serves as a lubricant. The mixture is then forced into the mold under pressure and the binder subsequently removed by heating. Finally, the resulting part is heated to its sintering temperature. While this process is an improvement over the first generation "press and sinter metallurgy" process, the resulting parts shrink considerably when heated to remove the binder from the preform, and there are limitations on the size of parts that can be fabricated by this process. Examples of this type of powder-injection molding include the following:
U.S. Pat. No. 4,113,480 discloses a method for injection molding parts formed from powdered metals by mixing the powder with a plastic medium comprising an organic binder and modifiers dissolved in a solvent. The solvent may be volatilized upon heating and the organic binder may be volatilized or sublimed during sintering. The disclosed preferred organic binder is methyl cellulose and the preferred solvent is water. Modifiers, which may be used to promote mold release and prevent the formation of cracks, include a combination of glycerin and boric acid.
U.S. Pat. No. 4,483,905 discloses a composition comprising a metal powder and a binding agent in solid or liquid state. The preferred binding agents are polyethylene glycols, polypropylene glycols, polyvinyl alcohol and glycerol.
U.S. Pat. No. 4,504,441 discloses a method for preventing segregation of powders having different specific gravities in a metal powder composition. The powdered metal and lubricant powder are mixed with furfuryl alcohol and an acid such as toluene sulfonic acid to convert the alcohol to a solid resin film on the powder metal particles. The polymerization of the alcohol may take place during mixing.
U.S. Pat. No. 3,539,472 discloses a process for producing molded metal powder articles by the use of mold-facilitating lubricants consisting of amides or diamides of aliphatic monocarboxylic acids and alcohols or diols or polyglycols. The lubricant mixtures may be burned out of the metal powder article formed by the process.
U.S. Pat. No. 4,906,424 discloses a method for injection molding ceramic or metallic powders by use of a binder. The ceramic powder may have a multimodal particle size distribution such that smaller diameter particles are provided to fill the interstices between larger particles. The primary binder material is a polymerized monomer or mixture of monomers which may be polymerized thermally, radiatively, or catalytically. The polymerized monomers may include various polyols. Suitable dispersants or surfactants such as oleic acid and stearic acid may be included in the binder as processing aids.
The mixture of binder and ceramic or metallic material may be injection molded. The mold temperature is maintained at about 50.degree. C. to about 200.degree. C. to initiate polymerization. The binder may be burned off by heating the preform to a temperature below about 700.degree. C. Finally, the molded article is sintered at a temperature ranging from approximately 700.degree. C. to 2200.degree. C. to obtain the final product.
The aforementioned prior art processes have two significant drawbacks. First, it is often difficult to form intricate or irregularly shaped parts because the powder compositions have inadequate flow properties which lead to density variations within the parts. In addition, the aforementioned processes often produce shrinkage and distortion of parts during heating phases, since binders are essentially totally removed without modifying the connecting structure between the powder particles.